Etymology

Yggdrasil Tree

Its form may be a reference to Norse mythology guardians of the Yggdrasil tree: a bird-like creature that protects the top of the tree, this may be Yveltal, a deer-like creature that protects the soil and trunk of the tree, this may be Xerneas, and a snake-like creature that protects the underworld below the tree is Zygarde.

Its name may come from the word tall since it can fly and start with the letter Y because it represents the Y axis in the 3D axis of X, Y and Z.